1. Field of the Invention
This invention relates to apparatus and method for electrically stimulating a spinal cord. More specifically, this invention relates to an apparatus and method for changing the intensity and location of resulting spinal cord stimulation by changing the pulse parameters of at least two separate voltage or current controlled sources applied to in line electrodes transverse to the spinal cord axis.
2. Description of the Prior Art
In epidural spinal cord stimulation (ESCS) two major practical problems reduce the efficacy of this therapy. One is the difficulty of directing the stimulation induced paresthesia to the desired skin areas and the other is the problem of motor responses to the stimulation, which reduces the amplitude range of the stimulation. It is generally agreed that in ESCS, for chronic pain, paresthesia should cover the whole pain region. With present stimulation methods and equipment only highly skilled and experienced surgeons are able to position the lead in such a way that the desired overlap is reached and desired results are obtained over time. It is difficult to focus the stimulation on the desired region during surgery and, with single channel approaches, impossible to refocus it afterwards, even though some small readjustments can be made by selecting a different contact combination, pulse rate, pulse width or voltage.
Especially the possibility of refocusing paresthesia after surgery would be highly desirable because, even if during surgery paresthesia covers the pain area perfectly, the required paresthesia pattern often changes later. This may be caused by such things as lead migration or histological changes, such as the growth of connective tissue around the electrode. The problem of lead placement has been addressed by U.S. Pat. No. 5,121,754 by the use of a lead with a deformable distal shape.
Using mathematical modeling we have discovered that the superposition of potential fields due to simultaneous stimulation by multiple pulse generators and connected electrodes will result in a significant change in the size and shape of the stimulated spinal cord area. This means that post-operative changes in stimulation fields can be obtained by selective parametric changes in the pulse generator outputs. Such changes in the stimulated spinal cord area will not only improve pain suppression but unwanted motor responses will be minimized or eliminated as well. These changes in stimulated area are impossible to obtain using a single channel stimulation.
U.S. Pat. No. 4,379,462 provides multiple electrodes but does not address the problem of post operative field changes and does not provide superimposed fields due to multiple channel stimulation.
U.S. Pat. No. 3,646,940 provides electrical means for locally stimulating masses of electrically excitable tissue using several pulse generators which are electrically connected to multiple electrodes at distant sites. The problem addressed includes bladder evacuation where an electrical pulse will contract the bladder but simultaneously contract the sphincter thus inhibiting evacuation. This problem is overcome by the use of a second time shifted electrical pulse to inhibit the sphincter response. This approach using separate bipolar electrodes to stimulate a nerve at multiple sites can not address the problem of the field superposition necessary to shift a stimulation field with respect to the spinal cord. This is because the stimulation sites according to this teaching are so far apart that the potential fields do not overlap, and thus will not give another field by linear superposition even if pulses are applied simultaneously to the two bipolar electrodes. Moreover, the precise and stable positioning of bipolar electrodes relative to each other necessary to establish desired and known field superposition is not obtainable by surgical implantation of separate electrode pairs. Therefore, this patent does not address the use of varying superimposed fields to vary the population of recruited nerve fibers.
The problems of directing stimulation induced paresthesia to desired skin areas, of unwanted motor responses to stimulation, of correcting for lead migration or incorrect positioning during surgery, and of making significant postoperative field changes have not been solved by existing apparatus and methods.